Obviously he’s into twins. Olivier Sarkozy, 42, the much older beau of elf-like Mary-Kate Olsen, 26, has plunked down $6.25 million for a storied East 10th Street townhouse, which, like the “Full House” star, has a twin. The half-brother of the former French president bought the pad alone, but plans to share the palatial 146-year-old love nest with Olsen, sources said. Sarkozy is buying it because he and Olsen “like that it is old,” a real estate insider told The Post’s Jennifer Gould Keil. The 4,200-square-foot home, built the year Abraham Lincoln took office, was designed by architect James Renwick Jr., best-known for St. Patrick’s Cathedral and the Smithsonian. Sarkozy paid the full asking price for the five-story house with nine fireplaces, a Juliet balcony in front and a large rear garden. The townhouse is in the St. Mark’s historic district, but the couple can gut the inside so long as they keep the façade intact. It isn’t clear how much renovation they plan to do, although the plumbing and kitchen need to be completely updated. Listing broker Jason Haber, CEO of Rubicon Property, declined to comment. The sale has brokers wondering if Mary-Kate’s sister Ashley will snap up the house’s twin, which was initially being sold as a package with its next-door neighbor but recently was taken off the market. Sarkozy — whose ex-wife, Charlotte, has blasted his May/December relationship with Olsen as “grotesque” — finally unloaded his Upper East Side townhouse earlier this year for $8.4 million by slashing the price from $11.95 million after it languished without a buyer. A rep for Olsen did not comment.